Doesn't the regulative principal exclude the use of musical instruments? Rap music is the music of criminals and the beat is jungle and reminiscent of paganism, why have the church imitate such nonsense in worship? Let's get our guitars, leather and hair spray out while we're at it. I loved heavy metal when I was a lost, wayward teenager, but put it away when I became saved because I knew what it was, satanic.
Apart from the obvious logical leaps inherent in your argument. No one has suggested rap as music for worship here.
The use of musical instruments in worship depends upon whether said instruments are an element or circumstance of worship.
And the beats used in rap are indeed descended culturally from African folk music . . . by way of the Negro Spiritual, Jazz, R&B, Blues, and Soul. One can also link that genealogy with Bluegrass, which is derived from it and Irish folk music.